Ultra-wideband (UWB) communications systems are normally defined as carrier-less communications systems wherein the bandwidth of the signal being transmitted, fB, is greater than or equal to 0.20 fc, where fc is the center frequency of the signal being transmitted. Additionally, the UWB communications system should have a minimum bandwidth of 500 MHz. Note that the definition for UWB communications systems and devices is as defined by the Federal Communications Commission (FCC) of the United States. UWB communications systems have been around for a great number of years, and the majority of them fall under one type of system, they modulate a stream of short-duration pulses (with an approximate duration which ranges from 0.2 nanoseconds (ns) to 2 ns), either in time (pulse position modulation (PPM)), amplitude (pulse amplitude modulation (PAM)), or phase angle (bi-phase modulation).
The generation of the short duration pulses is a task that is normally performed by the transmitter of the UWB communications system, usually once the data that is to be transmitted has been encoded, spread, scrambled, and otherwise ready for transmission. Typically, a pulse generator is used to generate a generic pulse of desired shape, magnitude, and duration. Then, the generic pulse is modulated using some modulation scheme, perhaps PPM, PAM, or bi-phase modulation, or some other modulation scheme, and modulated using the data that is to be transmitted.
The short duration pulses used in the UWB communications systems can be generated in several different ways, depending on the shape of the pulse. For example, if the pulse is a relatively simple square wave, a simple current source and a resistor can be used. The use of the current source and resistor combination is representative of a pulse generator that is an analog pulse generator. Alternatively, if the pulse is a more complex shape, then data points (samples) of the pulse may be stored in a memory and then provided to a digital-to-analog converter (DAC), which will produce the pulse. The storage of samples of the pulse and the use of the DAC is typical of a digital pulse generator.
One disadvantage of the prior art is that the use of a simple current source and resistor combination is not very flexible. Basically, the combination can be used to produce only one type of pulse. Therefore, if the need should arise that a different type of pulse is needed, differing in terms of shape, duration, magnitude, etc., then the combination may not be able to generate the different pulse.
A second disadvantage of the prior art is that the use of a memory to store the samples of the pulse to be generated can result in the need to store a large number of different sets of samples to generate different types of pulses at varying durations and magnitudes and polarities. This can result in the need to have a relatively large memory.
A third disadvantage of the prior art is that the use of stored samples to generate the pulses is the speed of the digital-to-analog converter, which may not be able to perform the conversion at a sufficiently fast rate.